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An introduction to “cron” on Linux November 21, 2010

Posted by Tournas Dimitrios in Linux.

On your computer, some processes will start automatically. In particular, these are the service processes your computer needs to do its work. Other processes are started manually. This means that you have to type a command at the command line  or click a tab on the gui -interface to start them. There is also a solution between these two options. If you need a certain task to start automatically at pre-defined intervals, you can use cron to do so.
There are two parts in cron.

  • First is the crond daemon . This process starts automatically on all computers and will check its configuration every minute to see whether it has to issue a certain task. By default, cron reads its master configuration file, which is /etc/crontab .
  • Secondly is the crontab .Users specify which jobs to run, and when to run them, by configuring a file known as the “cron table”, more often abbreviated “crontab” .

Cron assumes that the system is on continuously. If the system is not on when a task is scheduled it is not executed. Anacron is a task scheduler similar to cron except that it does not require the system to run continuously. It can be used to run the daily, weekly, and monthly jobs usually run by cron.To use the Anacron service, you must have the anacron RPM package installed. To determine if the package is installed, use the command rpm -q anacron.To use the cron service, you must have the vixie-cron RPM package installed.

Lets get started :The main configuration file for cron, /etc/crontab, contains the following lines: 

The first four lines are variables used to configure the environment in which the cron tasks are run. The value of the SHELL variable tells the system which shell environment to use (in this example the bash shell), and the PATH variable defines the path used to execute commands. The output of the cron tasks are emailed to the username defined with the MAILTO variable. If the MAILTO variable is defined as an empty string (MAILTO=""), email will not be sent. The HOME variable can be used to set the home directory to use when executing commands or scripts.

Each line in the /etc/crontab file has the format:

        ,---------------->  minute
	|  ,------------->  hour
	|  |  ,---------->  day of month
	|  |  |  ,------->  month (1=Jan,2=Febr,..)
	|  |  |  |  ,----> dayofweek(0=Sun,1=Mond,..)
	|  |  |  |  |  ,->  command to run
	|  |  |  |  |  |
	25 04 1  *  *  echo " Hello , how are you "
  • minute — any integer from 0 to 59
  • hour — any integer from 0 to 23
  • day — any integer from 1 to 31 (must be a valid day if a month is specified)
  • month — any integer from 1 to 12 (or the short name of the month such as jan, feb, and so on)
  • dayofweek — any integer from 0 to 7 where 0 or 7 represents Sunday (or the short name of the week such as sun, mon, and so on)
  • command — the command to execute. The command can either be a command such as ls /proc >> /tmp/proc or the command to execute a custom script that you wrote.

For any of the above values, an asterisk (*) can be used to specify all valid values. For example, an asterisk for the month value means execute the command every month within the constraints of the other values.

A hyphen (-) between integers specifies a range of integers. For example, 1-4 means the integers 1, 2, 3, and 4.

A list of values separated by commas (,) specifies a list. For example, 3, 4, 6, 8 indicates those four specific integers.

The forward slash (/) can be used to specify step values. The value of an integer can be skipped within a range by following the range with /<integer>. For example, 0-59/2 can be used to define every other minute in the minute field. Step values can also be used with an asterisk. For instance, the value */3 can be used in the month field to skip every third month.

Any lines that begin with a hash mark (#) are comments and are not processed.

Examples of crontabs

# record the memory usage of the system every monday
# at 3:30AM in the file /tmp/meminfo
30 3 * * mon cat /proc/meminfo >> /tmp/meminfo
#run custom script the first day of every month at 4:10AM
10 4 1 * * /root/scripts/backup.sh

As you can see from the /etc/crontab file, it uses the run-parts script to execute the scripts in the /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, and /etc/cron.monthly files on an hourly, daily, weekly, or monthly basis respectively. The files in these directory should be shell scripts.

If a cron tasks needs to be executed on a schedule other than hourly, daily, weekly, or monthly, it can be added to the /etc/cron.d directory. All files in this directory use the same syntax as /etc/crontab.

The cron daemon checks the etc/crontab file, the etc/cron.d/ directory, and the /var/spool/cron directory every minute for any changes. If any changes are found, they are loaded into memory. Thus, the daemon does not need to be restarted if a crontab file is changed.

Users other than root can configure cron tasks by using the crontab utility. All user-defined crontabs are stored in the /var/spool/cron directory and are executed using the usernames of the users that created them. To create a crontab as a user, login as that user and type the command crontab -e to edit the user’s crontab using the editor specified by the VISUAL or EDITOR environment variable. The file uses the same format as /etc/crontab. When the changes to the crontab are saved, the crontab is stored according to username and written to the file /var/spool/cron/username.

Using the crontab Command :

Users seldom manage their crontab file directly (or even know where it is stored), but instead use the crontab command to edit, list, or remove it :   crontab {-e | -l | -r}

Switch Effect
-e edit current file
-l list current file
-r remove current file

Special strings
Cron also offers some special strings:

string meaning
@reboot Run once, at startup.
@yearly Run once a year, “0 0 1 1 *”.
@annually (same as @yearly)
@monthly Run once a month, “0 0 1 * *”.
@weekly Run once a week, “0 0 * * 0”.
@daily Run once a day, “0 0 * * *”.
@midnight (same as @daily)
@hourly Run once an hour, “0 * * * *”.

Usage: “@reboot /path/to/execuable1” will execute /path/to/executable1 when the system starts. See “man 5 crontab” for more info.

Cron Jobs Alternatives :

Some hosting companies don’t allow access to cron, but you can find a lot of cron jobs alternatives which are free or paid. Here are some services:



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